Incorrect/triangulated texture when importing .obj file
The issues you find here are a combination of three distinct problems all related to the quality of the original models, and most likely not caused by Blender or import method.


You model has bad topology - Most likely caused by being apparently made by some sort of NURBS or Solid modeling application, or maybe abuse of Boolean operations. Not really easily fixed unless you remake it from scratch
Your model has overlapping geometry - Particularly on the rifle handle there seem to be some duplicate or overlapping almost coplannar surfaces. You may try to clean that up by hand
Your model has inconsistent normals - Easily fixed by selecting all your faces in Edit Mode and pressing Ctrl+N.


Also have in mind that this has nothing to do with materials or textures, as OBJ doesn't really support proper material importing or exporting